1. Field of the Invention
The present invention relates to a proximity sensor, and in particular to a proximity sensor for use in semiconductor lithographic applications.
2. Background Art
Many automated manufacturing processes require the sensing of the distance between a manufacturing tool and the product or material surface being worked upon. In some situations, such as semiconductor lithography, that distance must be measured with an accuracy approaching a nanometer.
The challenges associated with creating a proximity sensor of such accuracy are significant, particularly in the context of photolithography systems. In the photolithography context, in addition to the need to be non-intrusive as well as to precisely detect very small distances, the proximity sensor cannot introduce contaminants or come in contact with the work surface, typically a semiconductor wafer. Occurrence of either situation may significantly degrade or ruin the quality of the material surface or product being worked upon.
Different types of proximity sensors are available to measure very small distances. Examples of proximity sensors include capacitance gauges and optical gauges. These proximity sensors have serious shortcomings when used in lithographic projection systems because the physical properties of materials deposited on wafers may impact the precision of these sensors. For example, capacitance gauges, being dependent on the concentration of electric charges, can yield spurious proximity readings in locations where one type of material (e.g., metal) is concentrated. More generally, optical and capacitive methods are prone to errors due to significant interactions with layers beneath photoresist coatings. Another class of problems occurs when exotic wafers made of non-conductive and/or photosensitive materials, such as Gallium Arsenide (GaAs) and Indium Phosphide (InP), are used. In these cases, capacitance gauges and optical gauges may provide spurious results, and are therefore not optimal.
U.S. application Ser. Nos. 11/646,612 and 10/322,768, and U.S. Pat. Nos. 4,953,388 and 4,550,592, all of which are incorporated herein by reference in their entireties, disclose an alternative approach to proximity sensing through the use of a fluid sensor. In this application, the use of the word “fluid” includes the use of either liquid or gas forms of a substance. A typical fluid sensor contains a reference nozzle and one or more measurement nozzles to emit a fluid flow onto reference and measurement surfaces. Measurements are made of the back pressure differences within the sensors to determine the distance between the measurement nozzle and the measurement surface. A fluid sensor is not vulnerable to concentrations of electric charges or to the electrical, optical or other physical properties of a wafer surface. A fluid sensor detects only the top physical layer, and thereby yields a superior result. Accordingly, these types of sensors are ideal for topographic measurement of a material surface, such as that used to establish focus prior to lithographic exposure.
In order for proximity sensors (often called air gauges) to be utilized in EUV lithographic applications, such proximity sensors need to operate in a hard vacuum environment. Such vacuum conditions require the proximity sensor to use limited fluid mass flow rates in order not to overload the pumping systems used to maintain vacuum. The fluid dynamics in a conventional operating region yields proximity sensors with excess pressure gain, but a slower response time due to the lower fluid mass flow rates.
In other applications, proximity sensors function in atmospheric conditions. In such environments, proximity sensors are limited by how much flow can be pushed through a nozzle without incurring a noise penalty. In such environments, a shroud is often employed in an attempt to isolate the nozzle from environmental pressure changes. Typically, such atmospheric-based proximity sensors suffer from low gain and high noise compared to their vacuum-based counterparts described above.